As explained in more detail below, known vibrators of this sort can only be made cheaply with relatively small-diameter pistons, because of the absence of seals between the piston and the lateral wall of the hollow body, and, above all, can only perform relatively short strokes with respect to the distance between the end walls. Short strokes and small piston diameters combine to reduce the capacity of known vibrators of the above type to generate high-energy vibration each time the piston strikes the end walls.